The present invention relates to thickened brine solutions and, more particularly, to so called heavy brines having a density of about 14.2 pounds per gallon (ppg) or greater.
Hydroxyethyl cellulose (HEC) is used as a viscosifyer in aqueous solutions containing dissolved salt. Being non-ionic, it is one of the few materials which will hydrate in and efficiently viscosify heavy brine solutions of the type used as well servicing fluids, e.g. in workover and completion operations.
Heavy brine solutions having a density greater then about 15.0 ppg contain a soluble salt having a density greater than the density of calcium bromide. Usually, zinc chloride and/or zinc bromide is used, preferably zinc bromide. It is sometimes necessary to utilize a brine solution having a density in the range of about 14.2 ppg to about 15.0 ppg which has a lower temperature of crystallization than a similar density solution containing calcium chloride and calcium bromide. Such a solution can be formulated, from a density standpoint, using zinc bromide. However, we have found that if the zinc bromide concentration is less than about 20% by weight, the HEC will not gel or thicken the brine. Indeed, we have found that while it is possible to viscosify a brine solution containing no zinc bromide or a brine solution containing a high concentration of zinc bromide, if the two thickened solutions are admixed to produce a solution containing zinc bromide in less than about a 20% by weight concentration, the viscosity of the mixed brine will be essentially the same as if no HEC is present.